Contrast agents have been shown to increase the clinical utility of magnetic resonance imaging (MRI). However, a proton MRI contrast agent which remains in the intravascular space is currently not available. Such an agent may provide a noninvasive high resolution method to locate an acute hemorrhage and identify tissue infarction. Chromium (Cr) labeled RBCs have been shown to decrease the T1 of in vitro perfused tissue, making it a potential intravascular contrast agent. An animal model of acute GI hemorrhage will be utilized to evaluate the utility of this agent in identification of extravasation of blood from the intravascular space. Ten percent of the blood volume of a dog will be labeled with Cr, following which the blood will be pumped into the gastrointestinal (GI) tract at a fixed rate. The sensitivity of MRI to detect the site of GI hemorrhage will be evaluated for different locations of the alimentary tract and various rates of simulated hemorrhage. Changes in the T1 and T2 of normal canine tissue with infusion of Cr labeled RBCs will be studied to evaluate the feasibility of identifying infarction with this agent. T1 and T2 measurements of lung, liver, splenic and renal tissue, pre-and post-contract, will be made in vivo. The tissue which demonstrates the greatest change in T1 will be selected for identification of experimentally created infarction. Should T1 and T2 measurements of normal canine tissue following contrast enhancement indicate insufficient change of these parameters to identify infarction, Cr labeled red cells will be heat damaged to target the reticuloendothelial system for selected enhancement. Changes in T1 and T2 of normal canine liver and spleen, following infusion of heat damaged Cr labeled red cells, will then be evaluated. Infarcted spleens will be studied if sufficient contrast enhancement occurs with the use of heat treated cells in normal tissue. Lastly, the survival of Cr treated RBCs will be investigated. A small quantity of 51Cr will be included in the nonradioactive Cr utilized to label red cells. Periodic sampling of the dog's blood following labeling will yield a survival curve for cells treated in this manner. This investigation will provide important information concerning whether Cr labeled RBCs have the potential to become a clinically important MRI contrast agent.